Self-assembly of high-quality covalently bound organic monolayers onto silicon

A very mild method has been developed to obtain covalently attached alkyl monolayers from the attachment of 1-alkynes onto hydrogen-terminated silicon surfaces at room temperature in the dark. Apart from being the mildest method reported so far for the preparation of such monolayers, their quality, as indicated by water contact angles, XPS, and infrared spectroscopy, equals within experimental error that of the best reported alkyl monolayers on silicon.     

Metal-ion adsorption on carboxyl-bearing self-assembled monolayers covalently bound to magnetic nanoparticles

Hematite nanoparticles have been functionalized with a self-assembled monolayer of undecanoic acid (UA) by covalent attachment via siloxane groups. The number density of carboxylic/carboxylate groups on the surface was determined by titration, yielding values on the order of 10(14) UA molecules/cm(2) and thus consistent with a closed-packed monolayer. The ability of these functionalized particles to adsorb Cd(2+) in the ppm range was demonstrated using in situ anodic stripping voltammetry.     

Spectroelectrochemical detection of phenothiazine and phenoxazine derivatives covalently bound to self-assembled cystamine monolayers

The phenothiazine derivatives, Toluidine Blue O and Azur A, and the phenoxazine derivative Nile Blue were bound covalently to self-assembled cystamine monolayers chemisorbed on gold electrodes by derivatization of the surface amino groups with two different bifunctional spacers: terephthaloyl chloride and 1,6-hexamethylene-diisocyanate.The formation of the amido- and urea-derivatives of the parent compounds after covalent immobilization induces a shift of their redox potentials towards more positive values which can easily be detected by cyclic voltammetry.UV-Vis difference spectroelectrochemistry has been used to characterize the electroactive species immobilized onto transparent gold electrode surfaces in both oxidation states. In every case, the oxidized-minus-reduced (and reduced-minus-oxidized) difference spectra of the immobilized redox species show a shift of the UV maxima towards longer wavelengths and a shift of the Vis maximum towards shorter wavelengths when compared with their parent compounds. Each redox species showed different optical characteristics depending on the spacer used for immobilization.For phenothiazine derivatives immobilized with 1,6-hexamethylene di-isocyanate the total surface coverages obtained by optical methods were close to those obtained by cyclic voltammetry. However, for the same derivatives immobilized with terephthaloyl chloride, and for Nile Blue, independently of the spacer used, higher surface coverages were found by optical methods than by cyclic voltammetry.     

Electrocatalytical oxidation of hydroquinone with ferrocene covalently bound to L-cysteine self-assembled monolayers on a gold electrode

The preparation of a gold electrode modified by ferrocenecarboxylic acid (FcA) covalently bound to L-cysteine self-assembled monolayer (FcA-SAM) is described. The modified electrode shows an excellent electrocatalytic activity for the oxidation of hydroquinone (QH2) and accelerates the electron transfer rate. The anodic overpotential is reduced by ca. 290 mV compared to those obtained at bare gold electrodes. The charge transfer coefficient and the apparent surface electron transfer rate constant for the redox couple of Q/QH2 at the modified electrode are found to be 0.425 and 0.96 s(-1), respectively. The catalytic current response of DPV increases linearly with the QH2 concentration from 5.7 x 10(-7) to 3.2 x 10(-4) M. The estimation of QH2 in a simulative sample is satisfactory. The method is simple, quick, and sensitive.     

Single-step DNA immobilization on antifouling self-assembled monolayers covalently bound to silicon (111)

Hydrosilylation of alkenes with epoxide-terminated tri(ethylene oxide) moieties on Si-H surfaces yields homogeneous monolayers for the efficient coupling of biomolecules. The wetting properties of the epoxide-functionalized surface allow for the spotting of solutions of biomolecules, making the surface amenable to microarraying. Immobilization of thiolated DNA was achieved in a single step to fabricate biorecognition interfaces showing the hybridization of complementary DNA at low concentrations and negligible binding of noncomplementary DNA.     

Single-component and mixed ferrocene-terminated alkyl monolayers covalently bound to Si(111) surfaces

Self-assembled ferrocene monolayers covalently bound to monocrystalline Si(111) surfaces have been prepared from the attachment of an amine-substituted ferrocene derivative to a pre-assembled acid-terminated alkyl monolayer using carbodiimide coupling. This derivatization strategy yielded nanometer-scale clean, densely packed monolayers, with the ferrocene units being more than 20 A from the semiconductor surface. The amount of immobilized electroactive units could be varied in the range 2 x 10(-11) to approximately 3.5 x 10(-10) mol cm(-2) by diluting the ferrocene-terminated chains by inert n-decyl chains. The highest coverage obtained for the single-component monolayer corresponded to 0.25-0.27 bound ferrocene per surface silicon atom. The electrochemical characteristics of the mixed n-decyl/ferrocene-terminated monolayers were found to not depend significantly on the surface coverage of ferrocene units. The reversible one-electron wave of the ferrocene/ferrocenium couple was observed at E degrees ' = 0.50 +/- 0.01 V vs SCE, and the rate constant of electron transfer kapp was about 50 s(-1).     

Dispersion interactions enable the self-directed growth of linear alkane nanostructures covalently bound to silicon

Current interest in methods for controllably adding organic molecules to silicon surfaces relates to proposed hybrid silicon-organic devices. It was recently shown that a "self-directed" growth process, requiring only limited scanned probe intervention, has the potential to permit rapid, parallel production of ordered molecular nanostructures on silicon with predefined absolute position, structure, composition, and extent of growth. The hybrid organic-silicon structures formed are bound by strong covalent interactions. In this work, we use scanning tunneling microscopy and density functional theory techniques to show that molecule-surface dispersion interactions enable the growth process and play a crucial role in the final configurations of the nanostructures.     

Dielectrophoretic micropatterning with microparticle monolayers covalently linked to glass surfaces

Two-dimensional micropatterns of microparticles were fabricated on glass substrates with negative dielectrophoretic force, and the patterned microparticles were covalently bound on the substrate via cross-linking agents. The line and grid patterns of microparticles were prepared using the repulsive force of negative dielectrophoresis (n-DEP). The template interdigitated microband array (IDA) electrodes (width and gap 50 mum) were incorporated into the dielectrophoretic patterning cell with a fluidic channel. The microstructures on the glass substrates with amino or sulfhydryl groups were immobilized with the cross-linking agents disuccinimidyl suberate (DSS) and m-maleimidobenzoyl-N-hydroxy-succinimide ester (MBS). Diaphorase (Dp), a flavoenzyme, was selectively attached on the patterned microparticles using the maleimide groups of MBS. The enzyme activity on the patterned particles was electrochemically characterized with a scanning electrochemical microscope (SECM) in the presence of NADH and ferrocenylmethanol as a redox mediator. The SECM images proved that Dp was selectively immobilized onto the surface of microparticles to maintain its catalytic activity.     

Polyisobutylene containing covalently bound antioxidant moieties

Polyisobutylene (PIB) bearing covalently bound antioxidants is reported. TiCl₄‐catalyzed cleavage/alkylation reactions were conducted on poly(isobutylene‐co‐isoprene) (EXXON® Butyl 068; ${\overline{\mathrm{M}}}_{\mathrm{n}}$ = 3.37 × 10⁵ g/mol, (Ð) = 1.29, and 1.08 mol % isoprene) at ?70 °C in 60/40 hexane/dichloromethane in the presence of 2,6‐di‐tert‐butylphenol (DTP). Resulting PIB ${\overline{\mathrm{M}}}_{\mathrm{n}}$ s ranged from 30,000 to 85,300 g/mol and number average DTP functionalities (F_n) ranged from 4.3 to 12.0. ¹H NMR showed that 25%–40% of the DTP moieties underwent de‐tert‐butylation to form mono‐tert‐butyl phenol moieties. DTP‐functionalized and nonfunctionalized control PIBs were subjected to thermogravimetric analysis in nitrogen and in air. In nitrogen, commercial control PIBs (olefinic end groups) showed delayed onset of thermal degradation (T₁₀ = 380–381 °C) relative to both control PIBs produced by living polymerization (tert‐chloride end groups) and DTP‐functionalized PIBs (T₁₀'s all within the range of 366–370 °C). All PIBs showed lower degradation temperatures in air compared to nitrogen. Various control PIBs suffered 90% weight loss in air at temperatures ranging from 372 °C to 410 °C; DTP‐functionalized PIBs did not suffer 90% weight loss in air until 412–414 °C. Oxidative induction time analysis showed that all control PIBs suffered catastrophic degradation within 6 min, and most within 1 min, but DTP‐functionalized PIBs resisted degradation for >100 min. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 1836–1846     

Foldamer structuring by covalently bound macromolecules

We used fluorescence and electronic absorption spectroscopy to study the molecular weight dependence of macromolecule-induced folding in a chain-centered meta-phenylene ethynylene (mPE) oligomer. Analogous to the ability of intrinsically unstructured proteins (IUPs) to induce folding of globular proteins in cellular environments, we show that macromolecules attached to both ends of an mPE dodecamer induce the foldamer to collapse into a presumed helical conformation. The collapse is especially prominent once the macromolecule segments become larger than ca. 50 kDa. For sufficiently large macromolecules, the conformational structuring occurs even in solvents that normally denature the foldamer. Based on these findings, chain-centered foldamers might find use as models to investigate the fundamental macromolecular physics of IUPs.     

Unsupervised guided docking of covalently bound ligands

Summary An approach for docking covalently bound ligands in protein enzymes or receptors was implemented in MacDOCK, a similarity-driven docking program based on DOCK 4.0. This approach was tested with a small number of covalent ligand–protein structures, using both native and non-native protein structures. In all cases, MacDOCK was able to generate orientations consistent with the known covalent binding mode of these complexes, with a performance similar to that of other docking programs. This method was also applied to search for known covalent thrombin inhibitors in a medium-sized molecular database (ca. 11,000 compounds). Detection of functional groups suitable for covalent docking was carried out automatically. A significant enrichment in known active molecules in the first 5% of the database was obtained, showing that MacDOCK can be used efficiently for the virtual screening of covalently bound ligands.     

A small and stable covalently bound trianion

Stable doubly charged anions have become well known over the past decade, but the knowledge about higher-charged molecules is still sparse. Especially the minimum size of a covalently bound trianion which is still stable is an open question. Here, we present the smallest trianion of this kind known up to now, namely, B(C(2)CO(2))(3) (3-). After establishing its geometrical parameters, we investigate its stability with respect to electron autodetachment and fragmentation of the molecular framework. Our results lend strong support to the notion that this trianion indeed represents a stable compound which should be observable in the gas phase.     

Attractive mode manipulation of covalently bound molecules

We have investigated the manipulation of C₆₀ molecules on Si(1 0 0)-2×1 using the tip of a scanning tunnelling microscope. An attractive mode of manipulation has been identified in which molecules hop across the Si(1 0 0)-2×1 surface in steps of two lattice constants. An abrupt transition to repulsive manipulation occurs as the tip is lowered towards the surface. The attractive interaction is attributed to chemical forces between the tip and molecule.     

A novel family of ordered,mesoporous inorganic/organic hybrid polymers containing covalently and multiply bound microporous organic hosts

We have prepared a new family of periodic hybrid polymers containing microporous cavities provided by covalently bound organic hosts. Cyclodextrin (CD) or calixarene (CX) hosts are attached to four or more trialkoxysilyl groups, which are polymerized to form a polysilsesquioxane matrix. Structural integrity is provided by copolymerization with tetraethoxysilane, which produces a polysilicate co-matrix. Periodic order is created by carrying out the polymerization in the presence of a surfactant, cetyltrimethylammonium bromide. The resulting as-synthesized polymers from these three starting materials were characterized by solid-state nuclear magnetic resonance spectroscopy. The (13)C and (29)Si spectra provided evidence for intact polysilsesquioxane, polysilicate, organic host, and surfactant. Removal of the surfactant by washing produced a polymer containing cavities of mesoporous dimensions, in addition to the microporous host cavities. The purpose of introducing mesoporosity is to allow enhanced access of guests to the microporous hosts. Transmission electron microscopy demonstrated that both as-synthesized and solvent-extracted polymers have a periodic structure. All polymers are completely insoluble in water. The as-synthesized CD-containing polymers extracted up to >99% of 4-nitrophenol from aqueous solution, and the solvent-extracted CX-containing polymers extracted up to 67% of Fe(3+) and lesser amounts of other metal cations from aqueous solution, with interesting selectivity patterns. Simple filtration then removes the polymer containing the extracted organic molecule or metal cation. These extraction abilities are superior to previous materials.     

Displacement reactions of covalently attached organosilicon monolayers on Si

The covalently attached monolayers of alkylsilanes (R(CH(3))(2)SiX) on Si undergo complete displacement by the solutions of different organosilanes (R'(CH(3))(2)SiX). By varying the reaction time, the degree of displacement can be controlled offering a convenient method for the preparation of surfaces with mixed functionalities (R and R').     

Weak interactions between deposited metal overlayers and organic functional groups of self-assembled monolayers

The purpose of our research is to study the reactions, interactions or penetration between vacuum-deposited metals (M) and the organic functional end groups (OFGs) of self-assembled monolayers (SAMs) under controlled conditions. Metal/SAM systems are models for understanding bonding at M/organic interfaces and the concomitant adhesion between the different materials. In broad terms, the M/OFGs form interacting interfaces (e.g., Cr/COOH or Cu/COOH) in which the deposit resides on top of the OFGs or weakly interacting interfaces through which the overlayer penetrates and resides at the SAM/gold interface. We present a review of XPS results from weakly interacting systems (e.g., Cu/OH, Cu/CN, Ag/CH₃, Ag/COOH) and discuss in more depth the time-temperature dependence of the disappearance of the metal from the M/SAM interface following deposition. In this work, XPS and ISS were used to characterize octadecanethiol (ODT, HS(CH₂)₁₇CH₃), mercaptoundecanoic acid (MUA, HS(CH₂)₁₀COOH), and mercaptohexadecanoic acid (MHA, HS(CH₂)₁₅COOH) SAMs before and after depositing up to 1.0 nm Ag or Cu at ca. 10⁻⁷ torr. The SAMs were prepared by self-assembly onto gold films on <100> silicon substrates in an ethanolic thiol solution. XPS spectra indicate that no strong interaction occurs between the deposited Ag and the COOH organic functional group (OFG) of MUA or MHA, although a stronger interaction is evident for MHA, and a unidentate is formed for Cu on mercaptoundecanol (MUO). The Ag interaction with ODT is weak. ISS compositional depth profiles (CDPs) for Ag on MHA and MUA and ODT are compared over a temperature range of 113 to 293 K. The ISS results indicate that Ag remains on the surface of MUA for up to 1 h after deposition, whereas Ag penetrates ODT in less than 5 min at 295 K. The time for Ag to penetrate into MHA is several times longer than for MUA, depending on the SAM temperature. The time dependence of the slower Ag penetration through MUA and MHA is compared with that for ODT at temperatures below 295 K. Although Ag/OFGs are expected to have relatively weak interactions, the Ag/COOH system was anticipated to be more interactive than was found, so rapid penetration of Ag through the COOH SAM is an unexpected result.     

Synthesis and properties of water-soluble gold colloids covalently derivatized with neutral polymer monolayers

Citrate-capped gold nanoparticles as well as planar gold surfaces can be efficiently grafted with a covalently attached polymer monolayer a few nanometers thick, by simple contact of the metal surface with dilute aqueous solutions of hydrophilic polymers that are end-capped with disulfide moieties, as shown by UV/vis absorption, dynamic light scattering, and surface plasmon resonance studies. The hydrophilic polymer-coated gold colloids can be freeze-dried and stored as powders that can be subsequently dissolved to yield stable aqueous dispersions, even at very large concentrations. They allow for applying filtrations, gel permeation chromatography, or centrifugation. They do not suffer from undesirable nonspecific adsorption of proteins while allowing the diffusion of small species within the hydrogel surface coating. In addition, specific properties of the original hydrophilic polymers are retained such as a lower critical solution temperature. The latter feature could be useful to enhance optical responses of functionalized gold surfaces toward interaction with various substrates.     

Electron photodetachment dissociation of DNA anions with covalently or noncovalently bound chromophores

Double stranded DNA multiply charged anions coupled to chromophores were subjected to UV-Vis photoactivation in a quadrupole ion trap mass spectrometer. The chromophores included noncovalently bound minor groove binders (activated in the near UV), noncovalently bound intercalators (activated with visible light), and covalently linked fluorophores and quenchers (activated at their maximum absorption wavelength). We found that the activation of only chromophores having long fluorescence lifetimes did result in efficient electron photodetachment from the DNA complexes. In the case of ethidium-dsDNA complex excited at 500 nm, photodetachment is a multiphoton process. The MS³ fragmentation of radicals produced by photodetachment at λ = 260 nm (DNA excitation) and by photodetachment at λ > 300 nm (chromophore excitation) were compared. The radicals keep no memory of the way they were produced. A weakly bound noncovalent ligand (m-amsacrine) allowed probing experimentally that a fraction of the electronic internal energy was converted into vibrational internal energy. This fragmentation channel was used to demonstrate that excitation of the quencher DABSYL resulted in internal conversion, unlike the fluorophore 6-FAM. Altogether, photodetachment of the DNA complexes upon chromophore excitation can be interpreted by the following mechanism: (1) ligands with sufficiently long excited-state lifetime undergo resonant two-photon excitation to reach the level of the DNA excited states, then (2) the excited-state must be coupled to the DNA excited states for photodetachment to occur. Our experiments also pave the way towards photodissociation probes of biomolecule conformation in the gas-phase by Förster resonance energy transfer (FRET).     

Insights into the structure of covalently bound Fatty Acid monolayers on a simplified model of the hair epicuticle from molecular dynamics simulations

The epicuticle is the outermost layer of the human hair, and consists of a monolayer of fatty acids that is predominantly 18-methyleicosanoic acid (18-MEA) covalently bound to a protein matrix. Surprisingly, despite the clear scientific and industrial importance, the detailed molecular structure of this fatty acid layer is still poorly understood. In this work, we aim to gain insight into the structure of this so-called F-layer by performing molecular dynamics simulations on a simplified hair surface model consisting of a monolayer of 18-MEA covalently attached to graphene sheets at various separation distances. The relative free energy of the fatty acid layer was calculated as a function of separation distance in order to obtain the optimal packing density of the fatty acids. Conformational properties such as the thickness, tilt angle, and order parameter of the fatty acid layers were also calculated to characterize the structure of the F-layer. Simulations of the structurally similar eicosanoic acid (EA) were also performed as a comparison and to investigate the role of the anteiso-methyl side chain at the 18th position of 18-MEA. The degree of water penetration into the fatty acid layer at the various separation distances was also investigated. Our simulations suggest that the optimal spacing for the fatty acids is between 0.492 and 0.651 nm, in contrast to the generally accepted literature value of around 0.9-1.0 nm. This results in a packing density of between 0.21 and 0.37 nm(2) per fatty acid molecule and a thickness of around 2.01-2.64 nm. We also show that, at larger separation distances, the 18-MEA fatty acid provides a slightly better hydrophobic layer than the EA fatty acid, suggesting that the 18-MEA fatty acid may have been naturally selected to provide better protection for the hair when it loses some of the fatty acids due to daily wear and tear. To our knowledge, this is the first attempt to systematically investigate the hair surface structure and properties with molecular simulations.